The expected lifetime value of a custom acquisition is so high for these services that Groupon makes sense even if the conversion rate is < 5%. And, since most salon owners can push the risk downward to their commissioned (paid by the job, really) staff, there's even less disincentive. Only the staffers still trying to build a book of business will accept Groupon customers, and those employees' opportunity losses are low because they end-up sitting around between clients anyway. So, the risk/reward trade-off of using Grupon in these industries is quite good.
There seems to be way too many ads for spas, salons and the like, which do not interest me, yet they seem to attract tons of buyers..